A number of satellite communication systems are today in operation which allow users to communicate via satellite using only portable communication devices. These include the Global Positioning System (GPS) which provides positional and navigational information to earth stations, and telephone systems such as INMARSAT (TM). Demand for this type of personal communication via satellite (S-PCN) is expected to grow significantly in the near future.
One area which is of major importance is the development of a suitable antenna which can communicate bi-directionally with a relatively remote orbiting satellite with a satisfactory signal to noise ratio. Work in this area has tended to concentrate on the quadrifilar helix (QFH) antenna (K. Fujimoto and J. K. James, "Mobile Antenna Systems Handbook", Norwood, 1994, Artech House), pp. 455, 457. As is illustrated in FIG. 1, the QFH antenna 1 comprises four regular and identical inter-wound resonant helical elements 2a to 2d, centered on a common axis A and physically offset from one another by 90.degree.. In reception mode, signals received from the four helical elements are phase shifted by 0.degree., 90.degree., 180.degree., and 270.degree. respectively prior to combining them in the RF receiving unit of the mobile device. Similarly, in transmission mode, the signal to be transmitted is split into four components, having relative phase shifts of 0.degree., 90.degree., 180.degree., and 270.degree. respectively, which are then applied to the helical elements 2a to 2d.
The QFH antenna has proved suitable for satellite communication for three main reasons. Firstly it is relatively compact (compared to other useable antennae), a property which is essential if it is to be used in a portable device. Secondly, the QFH antenna is able to transmit and receive circularly polarised signals so that rotation of the direction of polarisation (due to for example to movement of the satellite) does not significantly affect the signal energy available to the antenna. Thirdly, it has a spatial gain pattern (in both transmission and reception modes) with a main forward lobe which extends over a generally hemispherical region. This gain pattern is illustrated in FIG. 2 for the antenna of FIG. 1, at an operating frequency of 1.7 GHz. Thus, the QFH antenna is well suited for communicating with satellites which are located in the hemispherical region above the head of the user.
A problem with the QFH antenna however remains it's large size. If this can be reduced, then the market for mobile satellite communications devices is likely to be increased considerably. One way to reduce the length of a QFH antenna for a given frequency band is to reduce the pitch of the helical elements. However, this tends to increase the horizontal gain of the antenna at the expense of the vertical gain, shifting the gain pattern further from the ideal hemisphere. Another way to reduce the length of the antenna is to form the helical elements around a solid dielectric core. However, this not only increases the weight of the antenna, it introduces losses which reduce the antenna gain.